The overall objectives of this program are: (1) to obtain information about the structures and catalytic mechanisms of dihydrofolate reductases, thymidylate synthetases from various sources (with particular reference to the inhibition of these enzymes, respectively, by amethopterin and fluorodeoxyuridylate), and to carry out similar mechanistic studies on ancillary folate coenzyme-dependent enzyme systems such as methionine synthetase folinate isomerase and seine hydroxymethylase; (2) to obtain information about the mechanisms by which folate compounds, particularly folate, 5-methyl tetrahydrofolate and amethopterin, are transported into cells; and (3) to utilize the above information in the design of new cancer chemotherapeutic regiments.